Research and development in the field of electronic equipment has emphasized the miniaturization of circuitry through the integration of discrete semiconductor components. Other areas of emphasis have included the development of circuitry utilizing higher clock speeds and baud rates to reduce semiconductor processing times. While these developments have led to smaller, more efficient and less expensive products, it has also resulted in products much more susceptible to disruption and damage from conducted electromagnetic interference (EMI) delivered by the AC power distribution network. The most visible impact of these trends is the proliferation of electronic, and particularly, microprocessor based equipment which now populates the power distribution network, nearly all of which is susceptible to the harmful effects of EMI.
An undesirable by-product of this rapid expansion in the use of electronic equipment is that the equipment itself is a source of EMI on the AC power network. This introduction of EMI to the AC network has become particularly acute with the expanded use of the switch mode power supply. Since it is clear that these trends will continue in the future, it is likely that the conducted EMI problem on the AC power distribution network will soon reach epidemic proportions.
Conducted EMI has many sources and thus takes a variety of shapes and forms. Typically, however, EMI is divided into two categories; radio frequency interference (RFI), which describes repetitive interference, and transients, which describe singular, short duration overvoltage conditions.
The sources of RFI range from radio communication systems, which can result in high frequency common mode interference (line-ground and neutral-ground propagation paths), to switch mode power supplies which generate high frequency normal mode interference (line-neutral propagation paths). A spectral analysis of the sources of RFI suggests that the lower limit of this interference is at approximately 30 KHz in the normal and common mode. Further, since the AC distribution network is a three conductor system having distributed resistance, inductance and capacitance, it has been found that the network exhibits a prominent low pass filter characteristic, resulting in an upper limit for AC main conducted RFI of approximately 30 MHz. Thus the spectral occupation of conducted RFI is typically in the 30 KHz to 30 MHz band. In view of this characteristic, the FCC and VDE have set limits on the conducted RFI emissions from digital electronic equipment which is to be connected to the power distribution network. These limits become stringent typically from 150 KHz to 30 MHz.
The effectiveness of RFI suppression networks can be tested in several ways. A test which has become the industry standard is the 50 ohm insertion loss test which is normally conducted through the use of a spectrum analyzer and tracking generator designed to operate over the frequency range in question. In this case the frequency band between 30 KHz and 30 MHz. The result of this test is an attenuation curve of the device being tested throughout the desired frequency range.
Conducted transients can be produced in the common mode by natural sources such as lightning strikes or in the normal mode by the switching action of large inductive loads. As would be expected from these varied sources, transients occurring in the power distribution network are widely varied with respect to both amplitude and duration, and thus, energy content. The exact characteristic of any given transient is a function of many variables, including the source of the transient as well as the characteristics of the distribution network itself.
In order to provide a testing standard for a reasonable worst case transient occurrence, the Institute of Electrical and Electronic Engineers (IEEE) has developed standardized test waveforms for various electrical environments. For indoor environments, the worst case transient condition is designated as the IEEE 587 Category B impulse waveform, defined in the 1980 IEEE publication "IEEE Guide for Surge Voltages in Low-Voltage AC Power Circuits". This impulse waveform, shown in FIG. 5, represents a worst-case waveform which is recommended for use in the testing of transient overvoltage protection circuitry. The standardization of this test waveform allows reproducible testing of transient protection devices.
The effects of the conducted EMI discussed above on electronic equipment connected to the power distribution network range from the disruption of normal operation to the damage of electronic components and circuits. Although EMI susceptibility levels vary as widely as do electronic equipment types, the trends in electronic equipment design have led to lower EMI tolerance levels and thus, a much more universal problem.